Order-Disorder Transitions in (Ca$_x$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$

TL;DR

This study uses single crystal x-ray diffraction to reveal that (Ca$_x$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$ undergoes an order-disorder structural transition characterized by persistent local atomic displacements across the phase change.

Contribution

It provides direct experimental evidence of order-disorder character in the structural phase transition of quasi-skutterudites using pair distribution functions.

Findings

Local atomic displacements are temperature-independent below the transition.

Displacements persist well above the transition temperature.

The results suggest an order-disorder nature of the phase transition.

Abstract

The classification of structural phase transitions as displacive or order-disorder in character is usually based on spectroscopic data above the transition. We use single crystal x-ray diffraction to investigate structural correlations in the quasi-skutterudites, (Ca$_x$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$, which have a quantum phase transition at $x\sim0.9$. Three-dimensional pair distribution functions show that the amplitudes of local atomic displacements are temperature-independent below the transition and persist to well above the transition, a signature of order-disorder behavior. The implications for the associated electronic transitions are discussed.